1. Field of the Invention
The present invention is related to a work vehicle, such as a wheel loader, forklift and so forth, and more particularly to a device for controlling vehicular steering.
2. Description of the Related Art
In a wheel loader, forklift or other such work vehicle, the steering mechanism of the vehicle is driven and controlled, and the direction of travel of the vehicle is changed in accordance with the operation of a controller, such as a steering handle, lever or the like.
(Prior Art 1)
FIG. 6 shows a conventional steering drive control hydraulic circuit, which is employed in a work vehicle. In this hydraulic circuit, pressure oil of a fixed delivery capacity is delivered from a fixed capacity-type hydraulic pump 22.
That is, a fixed capacity-type hydraulic pump 22 is driven, for example, by an engine 1. An oil line 23a is connected to the discharge opening of the fixed capacity-type hydraulic pump 22. This oil line 23a is linked to the input port of a flow control valve 36. The output port of the flow control valve 36 is linked to an oil line 23b. The oil line 23b is linked to an input port of the upstream side, as seen from the hydraulic pump 22, of a steering flow control valve 24. The steering flow control valve 24 has valve positions 24a, 24b, 24c. Valve position 24a is the valve position for supplying pressure oil to the one oil chamber 5a of a steering hydraulic cylinder 5 and for discharging pressure oil of the other oil chamber 5b to a tank 9; valve position 24b is the valve position for supplying pressure oil to the one oil chamber 5b of the steering hydraulic cylinder 5 and for discharging pressure oil of the other oil chamber 5a to the tank 9; and valve position 24c is the neutral valve position for shutting off the supply of pressure oil to the steering hydraulic cylinder 5. The steering flow control valve 24 is equipped with pilot ports 24d, 24e, and hydraulic signals S1, S2 corresponding to steering drive command signals are applied to each of the pilot ports 24d, 24e. When hydraulic signal S1 is applied to pilot port 24d, the steering flow control valve 24 is positioned on the side of valve position 24a, and when hydraulic signal S2 is applied to pilot port 24e, the steering flow control valve 24 is positioned on the side of valve position 24b. 
Input-output ports of the downstream side, as seen from the hydraulic pump 22, of the steering flow control valve 24 are linked, respectively, to oil chambers 5a, 5b of the steering hydraulic cylinder 5 via oil lines 23c, 23d. A tank port of the steering flow control valve 24 is linked to the tank 9 via oil line 23e. 
A rod of the steering hydraulic cylinder 5 is connected to the steering mechanism, and the steering mechanism operates in accordance with the telescopic motion of the rod of the steering hydraulic cylinder 5, changing the turning radius of the vehicle.
The operation of the steering drive control hydraulic circuit of FIG. 6 will be explained.
It is supposed that a steering controller, such as a steering handle, steering operating lever or the like was operated, and a steering drive command signal was generated. Here, a steering drive command signal is a signal that indicates an operators intention to change the orientation of the vehicle, and this signal is generated when an operation has been performed for changing the orientation of the vehicle from a straight forward state to a turning state, or when an operation has been performed for further increasing or decreasing turning from a steady turning state.
When a steering drive command signal is generated, either hydraulic signal S1 or S2 corresponding to this steering drive command signal is applied to either pilot port 24d or 24e of the steering flow control valve 24.
When hydraulic signal S1 is applied to pilot port 24d of the steering flow control valve 24, the steering flow control valve 24 is positioned on the side of valve position 24a. Thus, pressure oil delivered from fixed capacity-type hydraulic pump 22 is supplied to oil chamber 5a of the steering hydraulic cylinder 5 as pressure oil required by the steering flow control valve 24 by way of oil line 23a, flow control valve 36, oil line 23b, steering flow control valve 24 and oil line 23c. Also, unnecessary pressure oil in the flow control valve 36 is discharged to the tank 9 via an oil line 23p. Further, return pressure oil of oil chamber 5b of the steering hydraulic cylinder 5 is discharged to the tank 9 by way of oil line 23d, the steering flow control valve 24 and oil line 23e. In accordance with this, for example, the left-turn turning radius of the vehicle changes.
Further, when hydraulic signal S2 is applied to pilot port 24e of the steering flow control valve 24, the steering flow control valve 24 is positioned on the side of valve position 24b. Thus, pressure oil delivered from the fixed capacity-type hydraulic pump 22 is supplied to oil chamber 5b of the steering hydraulic cylinder 5 as pressure oil required by the steering flow control valve 24 by way of oil line 23a, flow control valve 36, oil line 23b, the steering flow control valve 24 and oil line 23d. Also, unnecessary pressure oil in the flow control valve 36 is discharged to the tank 9 via oil line 23p. Further, return pressure oil of oil chamber 5a of the steering hydraulic cylinder 5 is discharged to the tank 9 by way of oil line 23c, the steering flow control valve 24 and oil line 23e. In accordance with this, for example, the right-turn turning radius of the vehicle changes.
(Prior Art 2)
Further, as shown in FIG. 7, a steering drive control hydraulic circuit that carries out capacity control by using a variable capacity-type hydraulic pump 2 instead of a fixed capacity-type hydraulic pump 22 is also known in the art.
That is, the variable capacity-type hydraulic pump 2, for example, is driven by an engine 1. An oil line 33a is connected to the discharge opening of the variable capacity-type hydraulic pump 2. This oil line 33a is linked to an input port of the upstream side, as seen from the hydraulic pump 2, of a steering flow control valve 4. The steering flow control valve 4 has valve positions 4a, 4b, 4c. Valve position 4a is the valve position for supplying pressure oil to the one oil chamber 5a of a steering hydraulic cylinder 5 and for discharging pressure oil of the other oil chamber 5b to a tank 9; valve position 4b is the valve position for supplying pressure oil to the one oil chamber 5b of the steering hydraulic cylinder 5 and for discharging pressure oil of the other oil chamber 5a to the tank 9; and valve position 4c is the neutral valve position for shutting off the supply of pressure oil to the steering hydraulic cylinder 5. The steering flow control valve 4 is equipped with pilot ports 4d, 4e, and hydraulic signals S1, S2 corresponding to steering drive command signals are applied respectively to each of the pilot ports 4d, 4e. When hydraulic signal S1 is applied to pilot port 4d, the steering flow control valve 4 is positioned on the side of valve position 4a, and when hydraulic signal S2 is applied to pilot port 4e, the steering flow control valve 4 is positioned on the side of valve position 4b. 
Input-output ports of the downstream side, as seen from the hydraulic pump 2, of the steering flow control valve 4 are linked, respectively, to oil chambers 5a, 5b of the steering hydraulic cylinder 5 via oil lines 33c, 33b. A tank port of the steering flow control valve 4 is linked to the tank 9 via oil line 33d. 
A rod of the steering hydraulic cylinder 5 is connected to the steering mechanism, and the steering mechanism operates in accordance with the telescopic motion of the rod of the steering hydraulic cylinder 5, changing the turning radius of the vehicle.
A swash plate 2a of the variable capacity-type hydraulic pump 2 operates by moving in response to the movement of a capacity control valve 10. When the valve position of the capacity control valve 10 moves to the left side in the figure, the swash plate 2a of the variable capacity-type hydraulic pump 2 moves to the side of the minimum inclined rotation angle MIN, and when the valve position of the capacity control valve 10 moves to the right side in the figure, the swash plate 2a of the variable capacity-type hydraulic pump 2 moves to the side of the maximum inclined rotation angle MAX.
A spring 10a for applying a set pressure is disposed on the capacity control valve 10. The pressure of the downstream side of the steering flow control valve 4, that is, the load pressure PL of the steering hydraulic cylinder 5 can be detected as the pressure of outlet port 4f of the downstream side, as seen from the hydraulic pump 2, of the steering flow control valve 4. Outlet port 4f of the steering flow control valve 4 is linked by way of a pilot oil line 12 to a pilot port of the same side as the spring 10a of the capacity control valve 10.
The pressure of the upstream side of the steering flow control valve 4, that is, the delivery pressure Pp of the hydraulic pump 2 can be detected as the pressure inside oil line 33a. Oil line 33a is linked by way of a pilot oil line 11 to a pilot port on the opposite side of the spring 10a of the capacity control valve 10.
The operation of the steering drive control hydraulic circuit of FIG. 7 will be explained.
When a steering drive command signal is generated, either hydraulic signal S1 or S2 corresponding to this steering drive command signal is applied to either pilot port 4d or 4e of the steering flow control valve 4.
When hydraulic signal S1 is applied to pilot port 4d of the steering flow control valve 4, the steering flow control valve 4 is positioned on the side of valve position 4a. Thus, pressure oil delivered from the variable capacity-type hydraulic pump 2 is supplied to oil chamber 5a of the steering hydraulic cylinder 5 by way of oil line 33a, the steering flow control valve 4 and oil line 33c. Further, the return pressure oil of oil chamber 5b of the steering hydraulic cylinder 5 is discharged by way of oil line 33b, the steering flow control valve 4 and oil line 33d to the tank 9. In accordance with this, for example, the left-turn turning radius of the vehicle changes.
Further, when hydraulic signal S2 is applied to pilot port 4e of the steering flow control valve 4, the steering flow control valve 4 is positioned on the side of valve position 4b. Thus, pressure oil delivered from the variable capacity-type hydraulic pump 2 is supplied to oil chamber 5b of the steering hydraulic cylinder 5 by way of oil line 33a, the steering flow control valve 4, and oil line 33b. Further, return pressure oil of oil chamber 5a of the steering hydraulic cylinder 5 is discharged to the tank 9 by way of oil line 33c, the steering flow control valve 4 and oil line 33d. In accordance with this, for example, the right-turn turning radius of the vehicle changes.
The capacity control valve 10 controls the inclined rotation angle of the swash plate 2a of the variable capacity-type hydraulic pump 2, that is, the capacity, such that the differential pressure (Pp-PL) of the pump delivery pressure Pp that works via pilot oil line 11 and the load pressure PL of the steering hydraulic cylinder 5 that works via pilot oil line 12 matches the set pressure corresponding to the spring force of spring 10a. A flow corresponding to the aperture area of the steering flow control valve 4 is thereby supplied to the steering hydraulic cylinder 5 regardless of the load of the steering hydraulic cylinder 5.
The literature cited hereinbelow describes the general state-of-the-art related to the above-mentioned prior art 2.
Japanese Patent Application Laid-open No. 11-115780 discloses an invention, which provides a flow control valve for a working machine in addition to the steering flow control valve 4 shown in FIG. 7, increases the delivery capacity of the variable capacity-type hydraulic pump 2 (for steering) in accordance with the spool stroke (working stroke) of this working machine flow control valve, and supplies this increased portion to the working machine flow control valve.
Further, Japanese Patent Application Laid-open No. 6-117402 discloses an invention, which enhances the operating feel of the control lever regardless of the load of the hydraulic actuator by setting the maximum delivery quantity of a variable capacity-type hydraulic pump in accordance with the revolutions of an engine.
FIG. 3 shows the relationship between the spool stroke d of the steering flow control valve and the pump delivery flow Q in the above-described prior art 1 and prior art 2. Furthermore, it is supposed that this figure shows the relationship when the number of revolutions of the engine 1 is constant.
Further, FIG. 4 shows, on the time (t) base, the output response relative to input when a steering drive command signal St is inputted and the flow passing through the steering flow control valve (supply flow to the steering hydraulic cylinder 5) Q′ is outputted.
As shown in FIG. 3 (1), in the case of prior art 1, because a fixed capacity-type hydraulic pump 22 is used, a fixed maximum quantity pump delivery flow is delivered regardless of the spool stroked of the steering flow control valve 24. However, because a quantity of this fixed maximum quantity pump delivery flow that is in excess of the amount needed for steering drive is discharged to a tank 9 without being used for steering drive, energy loss is great.
As shown in FIG. 3 (2), in the case of prior art 2, because a capacity control valve is used, and the delivery flow Q of the variable capacity-type hydraulic pump 2 increases in accordance with an increase in the spool stroke of the steering flow control valve 4, the flow required for steering drive is delivered from the hydraulic pump 2 and is supplied to the steering hydraulic cylinder 5, resulting in extremely low energy loss.
Next, the responsiveness of the hydraulic pump relative to a steering operation will be explained by referring to FIG. 4.
In the case of prior art 1, as shown in FIG. 4 (1), a constant maximum flow is delivered from the fixed capacity-type hydraulic pump 22 regardless of the spool stroke d. Thus, if it is supposed that a steering controller, such as a steering handle, is rapidly moved at a timing of t1, the steering flow control valve 24 will operate pursuant to the generation of a steering drive command signal St and the flow Q′ supplied to the steering hydraulic cylinder 5 will rapidly increase. In other words, in the case of prior art 1, the output Q′ response in response to the input St, that is, the responsiveness of the steering control system is good.
By contrast, a capacity control valve is employed in the case of prior art 2. When a capacity control valve is used, the differential pressure (Pp-PL) across the steering flow control valve 4 varies pursuant to the generation of a steering drive command signal St. Then, the delivery capacity of the hydraulic pump 2 (inclined rotation angle of the swash plate 2a) varies in accordance with the differential pressure (Pp-PL) across the steering flow control valve 4. Then, the pump delivery quantity changes, and the supply to the steering hydraulic cylinder 5 changes in accordance with the change in the delivery capacity (inclined rotation angle of the swash plate 2a) of the hydraulic pump 2. The steering flow control valve 4 operates in accordance with the steering drive command signal St like this, and the inclined rotation angle of the swash plate 2a of the variable capacity-type hydraulic pump 2 changes in accordance with this operation, and the supply to the steering hydraulic cylinder 5 changes in accordance with this change in the swash plate inclined rotation angle. Therefore, the responsiveness of the steering control system is dependent on the responsiveness of the change in the delivery capacity of the variable capacity-type hydraulic pump 2 (change in the swash plate inclined rotation angle).
Here, the responsiveness of the variable capacity-type hydraulic pump is not as good as the responsiveness of the valves. In particular, there is a big time lag at the startup of operation of the variable capacity-type hydraulic pump. Thus, as shown in FIG. 4 (2), a delay occurs between the time a steering drive command signal St is generated and the swash plate 2a of the variable capacity-type hydraulic pump 2 begins operating, and the increase in the supply flow Q′ to the steering hydraulic cylinder 5 is delayed in accordance with this, causing the responsiveness of the steering control system to deteriorate compared to that of prior art 1.
FIG. 5 summarizes the points made hereinabove.
That is, in the case of prior art 1, energy loss is great, but the responsiveness of the steering control system is good. Conversely, in the case of prior art 2, energy loss is small, but the responsiveness of the steering control system is poor.
With the foregoing in view, it is an object of the present invention to reduce energy loss while at the same time enhancing the responsiveness of the steering control system.